This chapter investigates the forms and instruments of differentiation in the Area of Freedom, Security and Justice (AFSJ), and their impact on the idea of EU membership. As a particularly heterogenous and rapidly evolving area, AFSJ reveals a high level of differentiation, which is deeply entrenched in its policies, and has, by now, become constitutive. The chapter explores how differentiation is shaped in several AFSJ policies, including Schengen, police cooperation and cooperation in criminal matters (Europol and the European Public Prosecutor Office (EPPO)), and asylum. The analysis shows that the overlap of various forms of differentiation creates a space in which the distinction between participating and non-participating Member States, as well as external and EU members, gets diluted, and varies depending on the different participation patterns. The resulting landscape unveils an unravelling membership with considerable degrees of flexibility, and characterized by permeable and fraying edges. Finally, the analysis considers how Brexit is changing the very concept of EU membership, and what impact this is having on the differentiated integration of the AFSJ.